In Ray Bradbury's science fiction novel "Fahrenheit 451," that number represented the temperature at which books would burn, a symbol of a disturbing future under a totalitarian government.

For climate scientists, a similar number, 450 parts per million (ppm), holds its own ominous meaning. It represents a dangerous concentration of greenhouse gases in the atmosphere; a total that they were not expecting to be passed for at least another decade.

But a new UN-sponsored report, to be released next month, will show that as of 2005 the concentration of greenhouse gases in the atmosphere had already reached 455 ppm, according to Tim Flannery, a prominent Australian climate scientist who says he's seen the raw data that go into the document.

"We thought we'd be at that threshold within about a decade.... We thought we had that much time. But the new data indicates that in about mid-2005 we crossed that threshold.... What the report establishes is that the amount of greenhouse gas in the atmosphere is already above the threshold that could potentially cause dangerous climate change."

About 75 percent of the total ppm represents carbon dioxide, associated with burning fossil fuels. The rest is a combination of the other gasses, he said.

"450 ppm has long been held up as the threshold we dare not cross if we hope [to] avert the worst consequences of warming. Well, if Flannery is right, (and there's no reason to think otherwise) we crossed that line without even breaking stride."How did it happen? For one thing, countries such as China and India are actually "recarbonizing," Mr. Joseph says, meaning that their economies are becoming more energy-intensive "as they turn increasingly to [greenhouse-gas emitting] coal to feed their growth."

In May, the IPCC estimated current concentration of greenhouse gases at only 425 ppm, said a BBC report at the time. It noted that many scientists equated 450 ppm with a 2 degree C (3.6 degrees F.) rise in temperatures. Allowing temperatures to rise more than 2 C could lead to major impacts on the environment, scientists said. In the article, Rajendra Pachauri, the chairman of the IPCC, explained the strategy this way:

"If you want to stabilise around 450 ppm, that means in a decade or two you have to start reducing emissions far below the current level.... So in other words, we have a very short window for turning around the trend we have in rising greenhouse gas emissions. We don't have the luxury of time."

But, says Flannery, named Australian of the Year for 2007, that window is closed. According to the Australian Associated Press he says that higher figure is due to miscalculating the potency of other greenhouse gasses, which are included in the 450 ppm figure and measured in terms equivalent to that of CO2. But he adds:

A major UN climate change meeting in Bali, Indonesia, in December aims to set a course toward a new global agreement to reduce greenhouse-gas emissions. The current Kyoto Protocol, signed by the majority of the world's nations but not the United States, expires in 2012. Flannery told Reuters that the 450 ppm figure adds to the urgency and importance of that meeting.

"The longer we stay above the kind of levels we're at at the moment, the more likely it is that we would start to see the loss of the Great Barrier Reef; you would actually start see the collapse of the great ice sheets and places like the Amazon starting to burn down."•This weekly feature appears with links at csmonitor.com